This application is related to Japanese Patent Application No. 2001-261619 filed in Aug. 30, 2001, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a production method for an organic electroluminescent device, an electroluminescent device produced by such a production method, and an organic electroluminescent display panel employing such an organic electroluminescent device.
2. Description of the Related Art
Organic electroluminescent devices (organic EL devices) are light emitting devices typically including a first electrode, an organic film comprising at least a light emitting layer, and a second electrode, which are stacked in this order on a substrate. In recent years, the organic EL devices, which are expected to be applied to display panels, have been under intensive studies in various fields.
When a voltage is applied between the first and second electrodes of the organic EL device, electrons are injected into the light emitting layer of the organic film from one of the electrodes, and holes are injected into the light emitting layer from the other electrode. Thus, the electrons are coupled with the holes to provide facial light emission from the light emitting layer.
To provide the facial light emission, it is desirable that one of the electrodes is transparent. In most cases, a transparent substrate is employed as the substrate, and a transparent electrode film is formed as the fist electrode on the transparent substrate, so that the light emission can be outputted from the side of the first electrode. That is, the organic EL device includes the transparent first electrode, the organic film including at least the light emitting layer, and the transparent or non-transparent second electrode, which are stacked in this order on the transparent substrate.
A reason why the light emission is outputted from the first electrode side in the conventional organic EL device is that a glass substrate formed with an ITO conductive film for general use in a liquid crystal display panel and the like is used as the substrate for the organic EL device. The term xe2x80x9cITOxe2x80x9d stands for tin indium oxide, and the ITO film is generally employed as a transparent electrode.
The recent research and development is directed to an organic EL device which is adapted to output light emission from a second electrode side opposite from a substrate thereof for improvement of the characteristics and efficiencies of the organic EL device. More specifically, the organic EL device includes a transparent or non-transparent first electrode, an organic film comprising at least a light emitting layer, and a transparent second electrode, which are stacked in this order on a transparent or non-transparent substrate, so that facial light emission from the light emitting layer is outputted through the transparent second electrode.
This arrangement leads to improvement in light emitting efficiency and reduction in power consumption, because a loss in light emission can be suppressed which may otherwise occur due to reflection on the substrate in the case of the conventional device adapted to output the light emission from the first electrode side.
In addition, there is a wide choice of materials for the substrate. That is, a non-transparent substrate such as a ceramic substrate or a silicon substrate can be employed.
Where a display panel incorporating organic EL devices in pixel light emitting sections thereof is driven by TFTs (thin film transistors), the aperture ratio of the display panel is advantageously improved.
With reference to FIGS. 5 to 7, an explanation will be given to the construction of the organic EL display panel which includes conventional organic EL devices adapted to output light emission from a second electrode side, and to a production method therefor.
FIG. 5 is an explanatory diagram schematically illustrating the construction of the organic EL display panel (organic EL display) which includes the conventional organic EL devices adapted to output the light emission from the second electrode side. FIG. 6 is a diagram for explaining the step of forming organic films on first electrodes of the conventional organic EL devices shown in FIG. 5 by a transfer method, and FIG. 7 is a diagram for explaining the step of forming second electrodes on the transferred organic films.
The conventional organic EL display panel 110 shown in FIG. 5 includes organic EL devices 110a, 110b, 110c. The organic EL devices 110a, 110b, 110c each include a common substrate 101, a first electrode 102, an organic film 103 comprising at least a light emitting layer, and a second electrode 105.
For production of the organic EL display panel 110, the first electrode 102 and the second electrode 105 are each formed in a predetermined pattern. Where the organic EL display panel is of a full color type, the organic films 103 of the respective organic EL devices 110a, 110b and 110c should be formed separately for red (R), green (G) and blue (B) light emission.
The substrate 101 and the first electrodes 102 of the conventional organic EL devices 110a, 110b, 110c are not required to be transparent, but at least the second electrodes 105 on the respective organic films 103 are required to be transparent, because the light emission is outputted from the side of the second electrodes 105.
The formation of the organic films 103 on the first electrodes 102 is achieved by a known method such as an evaporation method, a spin coating method, a printing method, a laminating method or an ink jet method. In recent years, a transfer method has attracted attention.
In the transfer method, a donor film is prepared by forming a thin film (an organic film to be transferred onto the first electrode) on a base film such as a PET (polyethylene terephthalate) film by an evaporation method, a spin coating method or a sputtering method. The donor film is applied onto the substrate, and irradiated with an energy such as a laser beam or heat from the side of the base film thereof, whereby irradiated portions of the thin film are transferred onto the substrate (see, for example, Japanese Unexamined Patent Publications No. 9(1997)-167684 and No. 10(1998)-208881).
The base film is not limited to the aforesaid PET film but, where a laser beam is employed as a heat source, a PET film formed with a light-to-heat conversion layer and a heat conduction layer is preferably used.
An advantage of the transfer method is that layers of the thin film stacked on the base film are transferred onto the substrate as they are in a reverse stacking order. Therefore, the layers of the thin film (layers of the organic film) can collectively be formed on the substrate.
Further, there is no need to pattern the thin film on the base film, but it is merely necessary to form the thin film on the entire base film.
Since only the portions of the thin film irradiated with the laser beam or the heat are transferred from the donor film onto the substrate, the organic film can be formed in a highly precise pattern on the substrate without the use of a shadow mask as employed in the evaporation method. For example, the transfer method can easily form a line pattern having a width of 200 xcexcm or smaller. Therefore, a more precise organic EL display panel can more easily be produced than in the case where the conventional shadow mask is employed.
For the aforesaid reasons, the transfer method is advantageous for the production of the organic EL display panel.
With reference to FIG. 6, a more specific explanation will be given to the step of forming the organic films 103 of the conventional organic EL devices 110a, 110b, 110c shown in FIG. 5 by the transfer method.
As shown in FIG. 6, a donor film 210 including an organic film 103 provided over a base film 204 and comprising at least a light emitting layer is applied on a substrate 101 formed with first electrodes 102, and a heat source 205 is scanned over the donor film 210 perpendicularly to the first electrodes 102 for irradiation of the donor film 210 from the side of the base film 204. Thus, portions of the organic film 103 irradiated with the heat source 205 are transferred onto the first electrodes 102.
This step is repeated with the use of a donor film having a light emitting layer for red light emission, with the use of a donor film having a light emitting layer for green light emission, and with the use of a donor film having a light emitting layer for blue light emission, whereby the organic films 103 respectively adapted to emit red, green and blue light rays are transferred onto the first electrodes.
Then, transparent second electrodes 105 are formed on the respective organic films 103. Thus, the organic EL display panel 110 is produced as shown in FIG. 7.
As described above, most of the organic electroluminescent devices conventionally employ the ITO conductive film as the transparent first electrode formed on the substrate. The ITO conductive film is formed by an electron beam method or a sputtering method, and subjected to a heat treatment for improvement in the transmissivity and electrical conductivity thereof.
Where the transparent ITO conductive film is formed as the first electrode on the substrate by the aforesaid formation method, there is no particular problem. However, where the transparent ITO conductive film is formed as the second electrode on the organic film, the organic film underlying the second electrode is damaged because the organic film is less resistant to the electron beam method and the sputtering method. Therefore, the organic EL device cannot properly be produced.
Where the ITO conductive film is formed on the organic film by the sputtering method, for example, the organic film is easily damaged by a temperature increase due to the sputtering because of its lower heat resistance. Further, the organic film is damaged by impingement of evaporated material particles. In such a case, the organic EL device suffers from a leak current or a reduction in luminous characteristics. In the worst case, the organic EL device fails to provide light emission.
Particularly, where the organic film is formed by the transfer method, the heat resistance of the organic film is reduced as compared with the case where the organic film is formed by a vacuum evaporation method or a spin coating method. This is because the organic film is instantaneously subjected to heat in the transfer thereof. Therefore, the organic film is heavily damaged by the heat treatment performed after the ITO conductive film is formed as the second electrode by the sputtering method on the organic film formed by the transfer method.
It is known that the damage to the organic film can be suppressed by employing a transparent conductive film of In2O3xe2x80x94ZnO (IDIXO (registered trade name) available from Idemitsu Kosan Co., Ltd.) as the second electrode (see, for example, Japanese Unexamined Patent Publication No. 10(1998)-294182). In the case of the IDIXO transparent conductive film, there is no need for the post-sputtering heat treatment.
IDIXO comprises about 10 wt % of zinc oxide (ZnO) and the balance of indium oxide (In2O3), and has a sufficient electrical conductivity and a sufficient transmissivity even if the heat treatment is not performed after the sputtering.
However, the organic film is damaged to some extent as long as the second electrode is formed of IDIXO by the sputtering method.
In view of the foregoing, the present invention is directed to a production method for an organic EL device adapted to output light emission from the side of a transparent second electrode provided on an organic film, the production method featuring suppression of damage to the organic film which may be caused when the transparent second electrode is formed on the organic film by a sputtering method. The invention is further directed to an organic electroluminescent device produced by such a production method, and to an organic electroluminescent display panel employing such an organic electroluminescent device.
The present invention provides a production method for an organic electroluminescent device, the method comprising the steps of: forming a first electrode on a substrate; forming an organic film including a light emitting layer on the first electrode; forming an electrically conductive and light transmissive protection layer on the organic film; and forming a transparent second electrode on the protection layer by a sputtering method.
According to the present invention, the damage to the organic film which may be caused when the transparent second electrode is formed by the sputtering method is suppressed by forming the protection layer on the organic film and then forming the transparent second electrode on the protection layer by the sputtering method.